The present invention relates to actuating devices of the type comprising:
a controlled member that can be displaced from a first position into a second position, said first position corresponding to a resting position of the actuator;
a control member designed to actuate said controlled member, said control member being guided manually along a first actuation travel to reach an actuation position of the actuator;
a mechanical transmission that connects said control member to said controlled member, said mechanical transmission comprising a shape-memory element; and
electrical-supply means for supplying a current through said shape-memory element, said means being activated when said control member reaches said actuation position.
Shape-memory actuator (SMA) elements have been known from some time and used in a wide range of fields in which it is necessary to have available actuator means of a simple structure and of low cost. They use shape-memory metal alloys which can undergo deformation when a pre-set transition temperature is exceeded. In general, heating can be obtained in so far as the actuator element detects directly a variable environmental temperature, or else by supplying an electric current through the actuator element so as to heat it by the Joule effect.
In this case, the electrical-supply means can also be associated to electronic control means designed to control the current supply on the basis of a signal detected by a temperature sensor, by a position or displacement sensor, or else by a potentiometer.
Actuator devices of this type are, for example, used to provide manual-control actuation on motor vehicles, for example for actuating mobile parts or mechanisms of motor-vehicle seats, or for actuating mobile members of the engine or of services on board the motor vehicle.
The document No. EP 1 245 762 filed in the name of the holder of the present patent application describes a lock controlled via a control member. If said control member is brought into the actuation position, it activates electrical-supply means, which conduct a current through a shape-memory element constituted by a wire. Said wire is heated by the Joule effect and shortens in such a way as to open the lock.
A drawback of this actuating device lies in the fact that, when the shape-memory element is activated, this, by shortening, generates forces, in the direction of its length, which tend to bring the two ends of the wire closer to one another. Consequently, at the end connected to the control member a force is generated, which tends to displace the control member from the actuation position into the initial resting position. The user will then have to exert a force such as to cause the controlled member to remain in said actuation position. Even though the values of the forces involved are relatively small, this activation is in any case inconvenient for a user above all in the case where he does not expect any countering action.